1. Field of the Invention
This invention generally relates to a developing device for developing an electrostatic latent image formed on an imaging surface such as a photo-sensitive member for use in electrophotography, and particularly to a developing device for developing such a latent image by applying a thin film of charged toner particles thereto.
2. Description of the Prior Art
In general, in developing an electrostatic latent image using a single component developer, normally comprised of magnetic toner particles, in the field of electrophotography, it is required to form a thin film of uniformly charged magnetic toner particles prior to the application to the latent image. In one approach, use is made of a pressure member comprised of an elastic and magnetic blade, which is pressed against a developing sleeve, which is driven to rotate in a predetermined direction, across its full width, thereby forming a thin film of charged toner particles on the peripheral surface of the sleeve. In this case, it is desired that no such thin film of toner particles is formed in those portions to the non-image forming regions of the imaging surface on which the latent image is formed so as to reduce the waste of toner and to prevent contamination by floating toner within the machine. For this reason, the prior art approach was to provide seal members at those portions of the pressure member corresponding in position to the non-image forming regions thereby selectively preventing the thin film of toner particles from being formed on the developing sleeve. However, since such seal members are held between the pressure member and the developing sleeve under pressure, the thin film forming performance is directly affected by the conditions of these seal members, thereby making it difficult to form a desired thin film of toner particles stably at all times.